fix setCell

app/src/main/java/sudoku/Sudoku.java

@@ -31,8 +31,13 @@ public class Sudoku {
 				return false;
 			}
 		}
+		Cell cell = getCell(x, y);
+		if (cell instanceof MutableCell mCell) {
+			mCell.setSymbolIndex(value);
 			return true;
 		}
+		return false;
+	}
 
 	public Cell getCell(int x, int y) {
 		int index = y * getSize() + x;